This application is for continuation of support of studies in progress aimed at testing the hypothesis that a Ca2 ion-induced release of Ca2 ions from the sarcoplasmic reticulum is a step in the physiology of excitation-contraction coupling in mammalian cardiac muscle. The methods will use a preparation of skinned cardiac cells (sarcolemma removed by microdissection) that has been developed by the applicant, tension recording from these cardiac cells with a transducer sensitive in the microgram range together with a direct recording of change of (free Ca2 ions) with the Ca2 ions-sensitive probe, aequorin. In other experiments Ca2 ions transients will be recorded from myosin-extracted skinned cardiac cells in the presence of arsenazo III. A major part of the study to be done during the next year will be to use the direct detection of Ca2 ions transients to test hypotheses that had been inferred from previous studies that used only tension recording. The direct monitoring of the Ca2 ions transient is particularly important in the case where the experimental intervention pertubates the sensitivity of the myofilaments to Ca2 ions so that the tension developed by the myofilaments is no more a reliable sensor of the change of myoplasmic (free Ca ions). The studies will include: (1) Comparison between the timing of the Ca2 ions transients and that of the tension transients. (2) Comparison of Ca2 ions-induced release of Ca ions from the sarcoplasmic reticulum of various cardiac tissues, including the atrium and ventricle of the dog, cat, rabbit, rat, pigeon and frog and the Purkinje tissue of the dog. (3) Comparison between Ca2 ions transients and transients detected with potential-sensitive dyes. (4) Testing of the hypothesis that the Ca2 ions-induced release of Ca ions is graded with the (free Ca ions) used to trigger it. (5) Effect of length and of stretch and release on Ca2 ions transients.